For both-sided or multi-color printing of a web-shaped recording medium with toner images, U.S. Pat. No. 4,609,279 discloses that two printer devices be coupled via a turnover station with which the recording medium is turned over in a duplex mode or multi-color printing is performed on one side without turnover given a superimposed mode. A chemically acting cold fixing station is thereby employed as a fixing station for fixing the toner images on the recording medium.
The both-sided or single-side multi-color printing of the recording medium with two printer devices is significantly more problematical when these respectively comprise a thermal pressure fixing unit as disclosed, for example, by U.S. Pat. No. 5,179,417. One must thereby see to it that the fixing unit of the printer device arranged downstream does not heat the first toner image, which has already been fixed, to such an extent that it smears.
Another problem that has arisen when fixing with pressure and heat is the paper stress that occurs and the shrinkage of the paper connected therewith. Paper contains up to 10 weight percent water that evaporates given heating and thereby allows the paper to shrink, namely up to 2 mm in width given an A3 format. As long as the paper is printed on both sides, this is insignificant since a slight registration offset from the front side to the back side of the toner image is not disturbing. When a web-shaped recording medium is multiply printed on one side with the assistance of two printer devices without turnover and is thereby thermally fixed, the registration offset is extremely disturbing, particularly in what is referred to as a spot color mode.
A black toner image is thereby applied with a first printer and a chromatic toner image is then superimposed on this toner image with a second printer. By employing toner that can already be fixed at low temperatures, an attempt has been made to alleviate the paper stress and, thus, to enhance the registration precision. This, however, is cost-intensive and requires an exact regulation, especially in view of the fixing station.
For achieving high printing outputs with electrographic printer devices of 1,000 pages/minute and more, particularly for individualized mass printed matter like bills, brochures, etc., it is standard to print a paper web and to cut, sort and bind this with the assistance of a following post-processing unit. Single printers orxe2x80x94for the duplex or spot color modexe2x80x94two coupled printers were previously utilized for this purpose. Printer parks with a plurality of printers are thereby employed for handling high print volumes. These printer parks must be very flexibly designed with short conversion times in order to achieve a high usage degree.
Precisely in mass printing, it is increasingly necessary to employ an additional color for company logos or for emphasizing defined text parts, for example in invoices, in addition to monochromatic black-and-white printing. What is desired in invoice printing, for example, is a two-color printing on the front side with emphasis of the amount of the invoice in color and the explanations in black-and-white and, for example, instructions about legal recourse in black-and-white on the back side. Such print jobs have previously been handled in that two-color printing was first carried out on the front side with a color twin (two coupled printers) and this was rolled up on a take-up roller. The print job was subsequently resupplied to a single printer and the back side printer therewith. This is involved and leads to considerable problems in adhering to the registration precision.
It is therefore a goal of the invention to offer a printing system composed of a plurality of electrographic printer units that are coupled or can be coupled to one another that can be flexibly and quickly adapted to the greatest variety of print jobs.
Another goal of the invention is to design the printing system such that a web-shaped recording medium can be printed in multi-color fashion on the front side and at least single-color on the backside in one pass.
According to the method and apparatus of the invention, a method and system is provided for registered printing of a web-shaped recording medium with a plurality of functionally coupled, electrographic printer devices in a triplex arrangement. A front side of the recording medium is printed with a front side toner image in a first color and subsequent fixing is provided in a thermal fixing station with a first printer device. The printed recording medium is turned over in a turnover station. A backside of the web-shaped recording medium is printed with a backside toner image in one of a first and a second color and subsequently fixed in a thermal fixing station with a second printer device that accepts the turned over recording medium. The backside is additionally printed with an additional toner image and a color deviating from the backside toner image and subsequent fixing is provided in a fixing station with a third printer device.
The construction of a printer park that can be converted and flexibly adapted to the greatest variety of printing jobs is possible with the disclosed, variable triplex arrangement of printer chains having three serially coupled printer devices.
Surprisingly, multi-color printing on the front side with simultaneous single-color backside printing is also possible upon employment of printers having a thermal fixing when, in conformity with the invention, the backside is first printed in one color with the first printer device, is thermally fixed and turned over, the front side is then printed with toner of a first color with the following, second printer device, is thermally fixed, and the front side is then printed again with toner of a second color with the third printing device.
It has been shown that any percent of the maximum paper shrinkage already occurs in the initial thermal fixing. The further thermal fixing in the second printer causes no significant additional shrinkage. Nearly no registration offset between the toner images of the second and third printer thus occurs given the two-time printing of the front side. An additional registration control can further enhance the registration precision.
In order to alleviate the paper stress, it is advantageous to moisten the paper after the thermal fixing. The electrical charge behavior can thus also be governed better.
Embodiments of the invention are shown in the drawings and shall be explained in greater detail below by way of example.